refactor: rename frontend/ to web-player/ and format with prettier

Updates the compose build context, the Dockerfile header, the package
name and the docs. The compose service and container keep the name
frontend/musify-frontend, since nginx routes to http://frontend:3000.

The source files also carry the first prettier pass, so the rename and
the reformat land together for them.

web-player/src/routes/(app)/explore/+page.server.ts

@@ -0,0 +1,52 @@
+import type { PageServerLoad, Actions } from './$types';
+import { createApiClient, unwrapOrError } from '$lib/server/api';
+import { fetchYoutubeFiller } from '$lib/server/youtube';
+import { addTrackAction, addYouTubeToPlaylistAction } from '$lib/server/playlistActions';
+import { EXPLORE_PAGE_SIZE as PAGE_SIZE, YOUTUBE_FILLER_LIMIT } from '$lib/config';
+
+export const load: PageServerLoad = async ({ url, locals, fetch }) => {
+	const query = url.searchParams.get('q')?.trim() ?? '';
+	const page = Math.max(1, Number(url.searchParams.get('page')) || 1);
+
+	const api = createApiClient({ fetch, accessToken: locals.accessToken ?? undefined });
+
+	const tracksPromise = api.GET('/tracks', {
+		params: { query: { name: query || undefined, pageNumber: page, pageSize: PAGE_SIZE } }
+	});
+
+	const searchPromise =
+		query && locals.accessToken
+			? api.GET('/youtube/search', { params: { query: { query } } })
+			: Promise.resolve(null);
+
+	const playlistsPromise = locals.user
+		? api.GET('/playlists/users/{userId}', {
+				params: { path: { userId: locals.user.sub }, query: { pageNumber: 1, pageSize: 50 } }
+			})
+		: Promise.resolve(null);
+
+	const [tracksRes, searchRes, playlistsRes] = await Promise.all([
+		tracksPromise,
+		searchPromise,
+		playlistsPromise
+	]);
+
+	const tracks = unwrapOrError(tracksRes, 'No se pudieron cargar las canciones.');
+
+	return {
+		query,
+		tracks,
+		ytResults: searchRes?.data ?? null,
+		ytError: Boolean(searchRes?.error),
+		youtubeFiller: query
+			? Promise.resolve(null)
+			: fetchYoutubeFiller(api, locals.accessToken, YOUTUBE_FILLER_LIMIT),
+		playlists: playlistsRes?.data?.items ?? [],
+		needsAuth: !locals.user
+	};
+};
+
+export const actions: Actions = {
+	addTrack: addTrackAction,
+	addYouTubeToPlaylist: addYouTubeToPlaylistAction
+};